Objects are seen in three dimensions because light reflects from them and generates a light field in space. The two eyes of a viewer perceive this light field differently due to their different locations in space relative to the object, and the brain of the viewer processes the different perceptions of the light field by the two eyes to generate three-dimensional (“3-D”) perception. If a second light field (LF′) is artificially recreated that is the same as a first, original light field (LF), the viewer of LF′ will see the same object image in three dimensions. The basic quality of any 3-D imaging system therefore depends on the magnitude of the difference between LF and LF′, or, in other words, how close the imaging system can come to artificially recreating LF.
U.S. Pat. No. 5,745,197, issued to Leung et al, discloses a “volumetric” display intended to provide a type of 3-D imaging capability. As disclosed therein, the Leung et al. volumetric display creates viewable 3-D images that have real physical height, depth, and width by activating actual light sources at various depths within the volume of the display itself. In this manner, the two eyes of the viewer perceive various image elements at different depths within the volume of the display in perspective, thus creating a 3-D effect. The Leung et al. volumetric display utilizes a physical deconstruction of a 3-D object that entails “slicing” the object into pieces by planes oriented perpendicular to the view path of the viewer. Images corresponding to the resulting slices are then displayed superimposed on a stack of transmissive display screens (corresponding to the perpendicular slicing planes) layered at sequentially increasing distances from the viewer. The volumetric display thereby creates the appearance of a three dimensional image by reproducing individual cross sections of a contoured object on a series of screens wherein images on the screens closer to the viewer are stacked on top of more distant image pieces. Therefore, a three-dimensional effect is created in essentially a mechanical fashion. This type of volumetric display requires the layering of two or more transmissive imaging display panels to create the effect of depth, so its three-dimensional effect is limited necessarily by the depth, number and distance between the various display screens on which the image slices appear. Suitable display panels for this purpose include transmissive liquid crystal display screens.
Stereoscopic imaging is another technique utilized to simulate three-dimensional images to viewers. Stereoscopic displays operate by providing different yet corresponding perspective images of the same object or scene to the left and right eyes of the viewer. The viewer's mind thereby processes these two images to produce a perception of three dimensions. The principles of stereoscopic imaging have been applied to various areas for many years, including to the training of professionals, such as pilots to physicians, and to entertainment, such as 3-D movies and computer games. All stereoscopic systems rely upon one or more certain techniques to segregate images for the right and left eyes. Typically, stereoscopic imaging systems utilize special parallax barrier screens, headgear, or eye wear to insure that the left eye sees only the left eye perspective and the right eye sees only the right eye perspective.
U.S. Pat. No. 6,717,728, issued to Putilin et al. and commonly owned by the assignee of the present invention, discloses an autostereoscopic 3-D display that provides real-time and high resolution 3-D imaging capability without utilizing parallax barriers or specialized headgear. The Putilin et al. display utilizes an image processing algorithm to generate two or more calculated images from base stereopair images, which are the images that one ultimately wants to deliver to the two eyes of the viewer. A first one of those calculated images are sent to a distant display and the other one or more calculated images are sent to one or more transmissive displays placed in front (relative to the viewer position) of the distant display. Each display therefore simultaneously displays the calculated images that each contain at least some of the image information destined for each eye of a viewer. Each display's calculated image, when viewed simultaneously by a viewer, acts as a mask for and combines with the other displayed calculated images, resulting in the two different stereoscopic images being provided to the left and right eyes of the viewer, the stereoscopic effect being caused by the geometry of the spacing of the viewer's eyes and the spacing of the various layered displays. Putilin et al. discloses that the electronic processing to generate the calculated images necessary to deliver each of the base stereopair images to the appropriate eye can be accelerated by an artificial neural network. In one certain embodiments in the patent, multiple transmissive liquid crystal display panels are stacked one behind the other (relative to the viewer) in conjunction with a spatial mask, such as a diffuser, which is placed between liquid crystal displays to suppress Moiré patterns.
The layering of conventional passive or active matrix liquid crystal display (“LCD”) screens as utilized in the above patents is not optimal for purposes of 3-D display systems. A liquid crystal display is a thin, lightweight display device with no moving parts. It consists of a grid of pixel elements, with each pixel element including an electrically-controlled light-polarizing liquid trapped in cells between two transparent polarizing sheets. The polarizing axes of the two sheets are typically aligned perpendicular to each other. Each pixel is supplied with electrical contacts that allow an electric field to be applied to the liquid inside the corresponding cell or cells.
Before an electric field is applied, long, thin molecules in the liquid are in a relaxed state. Ridges in the top and bottom sheet encourage polarization of the molecules parallel to the light polarization direction of the sheets. Between the sheets, the polarization of the molecules twists naturally between the two perpendicular extremes. Light is polarized by one sheet, rotated through the smooth twisting of the crystal molecules, and then passes through the second sheet.
When an electric field is applied, the molecules in the liquid align themselves with the field, inhibiting rotation of the polarized light. As the light hits the polarizing sheet perpendicular to the direction of polarization, all the light is absorbed and the cell appears dark. In the relaxed state, however, the whole assembly appears nearly transparent to the eye. Between the two extremes, the cells also can be varied in increments to produce a grayscale effect.
The liquid crystal material used in standard LCD cells rotate all visible wavelengths equally, thus additional elements are utilized in standard LCDs to produce a color display. On common manner of providing a color LCD is to have each pixel is divided into three cells, one with a red filter, one with a green filter and the other with a blue filter. The pixel can be made to appear an arbitrary color by varying the relative brightness of its three colored sections. These color component cells can be arranged in different ways, forming a kind of pixel geometry optimized for the monitor's usage.
In all transmissive LCD panels, a slight darkening will be evident even in the relaxed state because of brightness losses from the backlighting source caused by the various sources, including the polarizing sheet for the backlight, the color filters, and the pixel grid materials. As individual transmissive LCD panels are stacked to produce 3-D displays, such as in the manner utilized in the Putilin et al. or Leung et al. displays, the brightness losses multiply, producing a less vivid and sharp 3-D display providing lower contrast and resolution. It would be desirable to preserve brightness in such 3-D display systems.
With regard to standard two-dimensional LCD panels, several technologies that create a single LCD panel by stacking two or three liquid crystal cells on top of one another have been developed in an attempt to maximize the quality of LCD images while reducing fabrication costs. U.S. Pat. No. 5,539,547, issued to Ishii et al, describes liquid crystal devices of the double-layer super twist nematic (“DSTN”) type that utilize plural polymer films. The DSTN type of LCD panel utilizes two transmissive passive-matrix LCD cells layered on top of one another to counteract the color shifting that occurs with conventional super twist passive matrix displays. Such DSTN displays are intended to be a more affordable and lower power-consumption alternative to thin film transistor (“TFT”) active-matrix LCD panels, but DSTN displays produce a lower quality picture than TFT displays. DSTN displays have double the response time (i.e., the lag time in forming screen graphics) of TFT displays, and typically only half the viewing angle capability. Contrast ratio (or picture sharpness) for DSTN displays also typically is significantly lower than for TFT displays, thus making DSTN displays generally undesirable for high quality graphic applications.
Phase change guest-host display (“PC-GHD”) screens provide an alternative to color filters for use in making full color LCD displays. Instead of providing color by incorporating three side-by-side cells and a color filter of three different colors for each individual pixel, PC-GHD screens layer three liquid crystal cells on top of one another for each pixel. Each of the three cells in the pixel has a different pigment added to its liquid crystal, and each cell can be varied in coordination with the other two cells from fully transmissive to fully blocking to produce any single color for the pixel. PC-GHD screens eliminate the color grid contained in standard TFT display panels, but PC-GHD screens are also largely undesirable for high quality graphics production as they currently provide lower contrast than TFT displays.
In light of the prior art in the field of 3-D imaging and transmissive display technology, it would be desirable to have a 3-D imaging system that provides high quality imaging of numerous aspects, perspectives or views to a given single user or multiple users in a dynamic manner. Such viewing optimally should take place in a flexible way so that the viewer is not constrained in terms of the location of the viewer's head when seeing the stereo image, and should provide a maximum of contrast and brightness to the viewer.